Reorder member functions in FecReceiver unit test file.

- Place all member function definitions between test fixture
  declaration and unit tests.
- Rename GenerateFrame -> PacketizeFrame.

No functional changes are intended by this CL.

BUG=webrtc:5654

Review-Url: https://codereview.webrtc.org/2275303003
Cr-Commit-Position: refs/heads/master@{#14488}

webrtc/modules/rtp_rtcp/source/fec_receiver_impl.h

@@ -46,6 +46,7 @@ class FecReceiverImpl : public FecReceiver {
   ForwardErrorCorrection::RecoveredPacketList recovered_packets_;
   FecPacketCounter packet_counter_;
 };
+
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_RTP_RTCP_SOURCE_FEC_RECEIVER_IMPL_H_